The present invention relates to additives and formulations which provide xe2x80x9cspecial effectxe2x80x9d appearances in plastic parts.
For a number of years, manufacturers have attempted to produce plastic articles, such as vinyl siding, with simulated wood-grain or multicolored appearance. Various methods of forming such plastic articles have been used, but none has succeeded in achieving a realistic wood-grain appearance in which the wood grain is strongly accented against the background. One known process for forming vinyl siding having some wood-grain appearance is shown in U.S. Pat. No. 4,048,041 to Nakamachi. The disclosure of U.S. Pat. No. 4,048,101 is hereby incorporated by reference. Nakamachi discloses a process for producing a synthetic plastic wood product with a multi-color appearance by forming colored particles of a first styrene resin with additives including a foaming agent, a foaming control agent, an inert inorganic foam nucleating material, and a dry colorant, and blending these colored particles with colored pellets of a second styrene resin having a lower melt flow index than the first resin. Nakamachi""s blended resins are then coextruded at which time the foaming agent increases the volume of the blended resins by 10% to 300%, and produces an annular ring pattern. U.S. Pat. No. 5,387,381, to Saloom also discloses a method for imparting a wood-grain appearance by coextrusion of a capstock including accent color pellets consisting essentially of acrylics and/or polycarbonates with a polyvinylchloride (xe2x80x9cPVCxe2x80x9d) substrate. The disclosure of U.S. Pat. No. 5,387,381 is also hereby incorporated by reference. Saloom""s lone example and only disclosed high deflection or softening point temperature acrylic resin, KMAX T-260, is no longer commercially available due to environmental concerns of its manufacturer. No polycarbonates and no other known mixture of acrylics has been found to yield even the marginal wood-grain effect achieved by KMAX T-260. Moreover, the method disclosed in Saloom results in somewhat unpredictable results because of the difficulty in simultaneously controlling the melt flow index and deflection or softening point temperature of KMAX T-260. KMAX T-260 is a resin having a high deflection or vicat softening point temperature, and a low melt flow index. The Saloom method does not result in bold contrasting or xe2x80x9chardxe2x80x9d streaking.
Furthermore, a continual need exists for decking products with improved durability and aesthetic qualities. Thus, there remains a need in the industry for a method of making simulated wood-grain plastic decking that gives predictable, controllable results in a pre-selected array of shadings and boldness of streaking, in which the decking planks display a realistic wood-grain appearance, strongly accented against the background.